1. Technical Field
The present invention relates to an apparatus for inflating an inflatable vehicle occupant protection device.
2. Description of the Prior Art
An inflatable vehicle occupant protection device, such as an air bag, is deployed upon the occurrence of a vehicle crash. The air bag is part of a vehicle occupant protection apparatus which further includes a crash sensor and an inflator. The inflator includes a housing and an inflation fluid source, such as a solid propellant, in the housing. When the crash sensor senses a crash-indicating condition of at least a predetermined threshold level, the inflator is actuated and produces inflation fluid under pressure in the inflator housing. The pressurized inflation fluid is directed out of the inflator housing and inflates the air bag into the vehicle occupant compartment. When the air bag is deployed in this manner, it helps to protect an occupant of the vehicle from a forceful impact with parts of the vehicle as a result of the crash.
When the inflator is actuated at an elevated ambient temperature, the pressure of the inflation fluid in the inflator housing increases. An inflator must be strong enough structurally to contain these elevated pressures. If the pressure in the inflator housing is thus increased, the mass flow rate of the inflation fluid flowing into the air bag can increase above the desired flow rate. Also, the possibility of such increased pressures may make it unfeasible to use a solid propellant which has a high burn rate exponent, that is, a high sensitivity to pressure variation.
The present invention is an inflator for providing inflation fluid for inflating an inflatable vehicle occupant protection device. The inflator comprises a housing. A primary propellant in a primary chamber in the housing is ignitable to provide inflation fluid to inflate the inflatable device. A secondary propellant in a secondary chamber in the housing is ignitable to provide inflation fluid to inflate the inflatable device. The inflator includes initiator means for selectively igniting either the primary propellant alone or both of the primary and secondary propellants. The inflator also includes a secondary propellant cap for closing the secondary chamber and for maintaining the secondary propellant in the secondary chamber. The secondary propellant cap has a first position blocking flow of combustion products of the first propellant into the secondary chamber when only the primary propellant is ignited. The secondary propellant cap is movable by combustion products of the secondary propellant when ignited from the first position to a second position enabling flow of inflation fluid out of the secondary chamber to inflate the inflatable device.